Facile mechanochemical synthesis of Co?NC catalysts for oxidative esterification of benzyl alcohol with methanol

Article abstract of DOI:10.1016/j.catcom.2020.105952

A robust and solvent-free mechanochemical approach for the synthesis of cobalt–nitrogen–carbon nanocomposite by simple mixing of cobalt(II) salts, nitrogen-containing ligands and carbon black, followed by thermal treatment is suggested. The catalysts obtained were tested in the oxidative esterification of benzyl alcohol, demonstrating highly competitive performance.

Full text of DOI:10.1016/j.catcom.2020.105952

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Facile mechanochemical synthesis of Co@NC catalysts for
oxidative esterification of benzyl alcohol with methanol
Tatiana V. Astrakova, Vladimir I. Sobolev, Konstantin Yu.
Koltunov
PII:
S1566-7367(20)30028-5
DOI:
https://doi.org/10.1016/j.catcom.2020.105952
CATCOM 105952
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
13 December 2019
31 January 2020
4 February 2020
Please cite this article as: T.V. Astrakova, V.I. Sobolev and K.Y. Koltunov, Facile
mechanochemical synthesis of Co@NC catalysts for oxidative esterification of benzyl
alcohol with methanol, Catalysis Communications (2019), https://doi.org/10.1016/
j.catcom.2020.105952
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© 2019 Published by Elsevier.

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Facile mechanochemical synthesis of Co@NC catalysts for oxidative
esterification of benzyl alcohol with methanol
Tatiana V. Astrakovaa,b, Vladimir I. Soboleva and Konstantin Yu. Koltunova,b,*
a Boreskov Institute of Catalysis, Russian Academy of Sciences, Pr. Akademika Lavrentieva, 5,
Novosibirsk, 630090, Russia
b Novosibirsk State University, Pirogova, 2, Novosibirsk, 630090, Russia
* Corresponding author
Konstantin Yu. Koltunov
Boreskov Institute of Catalysis, Russian Academy of Sciences,
Pr. Akademika Lavrentieva, 5, Novosibirsk, 630090, Russia
E-mail: koltunov@catalysis.ru
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Abstract
A robust and solvent-free mechanochemical approach for the synthesis of cobalt–nitrogen–
carbon nanocomposite by simple mixing of cobalt(II) salts, nitrogen-containing ligands and
carbon black, followed by thermal treatment is suggested. The catalysts obtained were tested in
the oxidative esterification of benzyl alcohol, demonstrating highly competitive performance.
Keywords: Cobalt-nitrogen-carbon catalyst; mechanochemical synthesis; heterogeneous
catalysis; oxidative esterification.
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1. Introduction
Carbon materials doped with transition metals (Me = Fe, Co, Ni, Cu, etc) and nitrogen,
usually denoted as Me@NC, represent extraordinarily active non-precious metal catalysts which
are used in a variety of newly developed green, economic and sustainable catalytic processes.
The latter involve hydrogenation of nitroarenes [1-3], reductive amination of carbonyl
compounds [4,5], selective oxidation and oxidative esterification of alcohols [6-12], oxidation of
amines [13,14], epoxidation of alkenes [15], oxidation of ethylbenzene to acetophenone [16],
hydrogen production via electrolysis [17], electrochemical conversion of molecular oxygen [18-
22], selective dehydrogenation of formic acid (for hydrogen fuel cells) [23] and also other
important applications [24,25].
In particular, cobalt-containing composites, Co@NC have shown the best catalytic
performance in the oxidative esterification of benzyl alcohol with methanol under quite mild
o
conditions, ca. 1 atm O2 and 20-60 C [6-9,26], which in fact can be used for testing the relative
catalytic activity of Co@NC materials obtained by different methods.
The most active Co@NC catalysts are commonly produced by the incipient wetness
impregnation of carbon black (or carbon nanotubes) with
Co(II) complex with 1,10-
phenanthroline precursor compound, followed by pyrolysis of the resulting material at 800 °C
under inert gas conditions [2,6,23]. Another usual technique is the carbonization of cobalt–
nitrogen containing metal-organic frameworks (MOFs), such as ZIF-67 [8,9,19-21] and others
[7,24] at temperatures in the 600 - 900 °C range, which ensures the formation of Co-N doped
graphitic structures. Besides, several other more or less sophisticated approaches toward
Co@NC formation have been elaborated [7,16,27-29].
In general, it is considered that the active sites of the Co@NC catalysts could be: a)
nanoparticles of cobalt oxide (e.g. Co3O4) incorporated into a graphite-like framework with
nitrogen-containing heterocyclic fragments [6]; b) nanoparticles with a core of metallic cobalt
surrounded by an oxide layer which is embedded in a graphene layer matrix [4]; c) atomic-size
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or subnanosize cobalt particles forming chelate complexes with pyridine-like vacancies in N-
doped graphene [2,3].
Currently, there is a dominant perception that highly dispersed single-atom Co centers
coordinated to nitrogen (CoNx sites) or cobalt particles surrounded by a protective graphitic layer
play a key role in the catalytic activity of Co@NC materials [16,17,23,25,27,28]. Accordingly,
the concurrent formation of Co nanoparticles as part of the catalyst is not really necessary for
high catalytic activity [23,27]. It is, therefore, not surprising that Co@NC composites obtained
with different approaches can show surprisingly similar catalytic behavior, even though the
relative content of cobalt nanoparticles (of the same size distribution) varies widely, ca. from 3 to
30 wt% [6,9]. Indeed, this is more likely to happen when highly dispersed Co or CoNx centers
supported on carbon consist the real active sites. Furthermore, in recent years, a great deal of
research effort has been devoted to the development of environmentally benign and cost-
effective procedures of fabricating catalytic materials. Among them, the elaboration of
mechanochemical methods for the preparation of catalysts has experienced a significant
evolution [30-32]. Mechanochemistry has developed as quite promising and simple methodology
able to compete with traditional synthetic protocols for MOFs, nanomaterials and catalysts
syntheses, which generally involve multi-step processes.
In this study, we report a rather technologically convenient, solvent-free and
green
way to synthesize Co@NC composites by the physical mixing (mechanochemical
grinding) of solid starting components, such as cobalt(II) salts, nitrogen-containing
ligands and carbon support, followed by
thermal treatment. Despite this fairly simple
preparation, the obtained materials had displayed a remarkable catalytic activity, well-
compared with that of the state-of-the-art Co@NC catalysts [6,9] in the oxidative
esterification of benzyl alcohol, which is used here as a probe reaction.
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2. Experimental
2.1. Catalysts preparation
(i) Co@NC-Phen. A solution of Co(OAc)2.4H2O (25 mg, 0.1 mmol) and 1,10-
phenanthroline monohydrate (46 mg, 0.2 mmol) in 10 mL of ethanol was stirred for 30
min at room temperature. Then, 138 mg of VulcanXC72R carbon powder (provided by
Cabot Ltd) was added to form a suspension, which was heated under reflux with agitation
for 4 h. After cooling the mixture to room temperature, the solvent was removed in
vacuum, and the solid obtained was further dried at 60 °C/5 Torr for 12 h, ground to
powder, and then annealed at 800 ± 10 °C (heating rate 5 °C min–1) for 2 h under flowing
Ar gas at 10 cm3 min−1 (cf. ref. [6]).
(ii) Co@NC-ZIF. ZIF-67 was obtained in the form of a blue-violet finely crystalline
powder via the reaction of 2-methylimidazole (97%, Alfa Aesar) with cobalt(II) nitrate
hexahydrate (extra pure, SLR, Fisher Chemical) by the procedure described earlier [9,26].
A Co@NC-ZIF sample in the form of a homogeneous black powder possessing
paramagnetic properties was produced by carbonization of ZIF-67 at 850 ± 10 °C
(heating rate 5 °C min–1) in a flow of argon gas in the course of 2 h.
(iii) Co@NC-Gr1-6,8,9 (typical procedure). A mixture of cobalt(II) salt (1 mmol scale;
see Table 1 for reagent loadings) and VulcanXC72R carbon powder was thoroughly
hand-ground with a pestle in an open mortar at room temperature for 10 min. Then, the
corresponding nitrogen ligand was added to the mass obtained, and the grinding was
continued for additional 10 min until the mixture became moist and turned eventually into
a homogeneous paste. The reaction mixture was dried under air flow in the hood within a
few hours. The resultant material was placed in a ceramic crucible and heat treated under
conditions indicated in Table 1 to offer the final material.
(iv) Co@NC-Gr7. A mixture of Co(NO3)2.6H2O (291 mg,
1 mmol) and 2-
methylimidazole (164 mg, 2 mmol) was hand-ground with a pestle in an open mortar at
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room temperature for 20 min to afford a bright purple wet residue which was left to dry
under a fume hood overnight. The obtained material was mixed with VulcanXC72R (455
mg), and the process of grinding was continued for another 20 min using a FRITSCH
Mortar Grinder Pulverisette 2 (Germany) equipped with agate bowl and pestle (contact
pressure ~23.8 daN), followed by heat treatment.
2.2 Catalysts characterization techniques
(i) Brunauer–Emmett–Teller (BET) specific surface area.
parameters were determined from N2 adsorption isotherms at 77 K on ASAP-2400
(Micromeritics, USA) analyzer after degassing the samples at 150 °C to a residual
The porous structure
pressure of 30 mTorr (4 Pa). The total accessible surface area by the BET method, and the
total volume of pores with effective sizes up to 100–200 nm (Vtotal) were estimated from
the amount of adsorption at the relative nitrogen pressure of ~ 0.99. A micropore volume
and surface areas of mesopores and micropores were calculated by the standard software
provided by the instrument.
(ii) Powder X-ray diffraction. A powder X-ray diffraction (XRD) analysis was
performed on a Siemens D500 instrument with CuKα radiation and θ–2θ focusing
geometry. The instrument was equipped with a graphite monochromator in the reflected
beam, which made it possible to diminish the contribution from the cobalt fluorescence
under the copper radiation. A scintillation detector was used for signal detection.
Measurements were made in the scanning mode at 2θ angles in the 10–70° range at a step
of 0.05° and accumulation duration at a point of 3 s. The analysis was made with a Bruker
EVA software package contained in the hardware-software complex. The X-ray
diffraction patterns were interpreted by the aid of
Diffraction Data) PDF-2 database.
the ICDD (International Center for
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(iii) High-resolution transmission electron microscopy (HRTEM).
Micrographs were
obtained with JEM-2010 (JEOL, Japan) instrument with lattice resolution 0.14 nm and
accelerating voltage 200 kV. The samples were prepared by ultrasonic dispersing in
ethanol and consequent deposition of the suspension upon a "holey" carbon film
supported on a copper grid. Local elemental analysis was performed with the EDX
method on an Energy-dispersive X-ray spectrometer «QUANTAX 200-TEM» (Bruker,
Germany) equipped with XFLASH detector with energy resolution  130 eV.
(iv) X-ray fluorescence (XRF). The content of Co element in the samples was
measured by XRF using a sequential spectrometer ARL Perform'X with a Rh anode X-
ray tube.
(v) X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectra were
recorded on
a
SPECS (Germany) photoelectron spectrometer using a hemispherical
PHOIBOS-150-MCD-9 analyzer (Al Kα radiation, hν = 1486.6 eV, 150 W). The binding
energy (BE) scale was pre-calibrated using the positions of the peaks of Au4f7/2 (BE =
84.0 eV) and Cu2p3/2 (BE = 932.67 eV) core levels. The binding energy value of peaks
was corrected to
consider the sample charging by referencing to the C1s (284.5 eV,
internal standard). The sample in the form of powder was loaded onto a conducting
double-sided copper scotch. The survey spectrum and the narrow spectra (C 1s, N 1s, O
1s and Co 2p) were registered at the analyzer pass energy of 20 eV. Atomic ratios of the
elements were calculated from the integral photoelectron peak intensities, which were
corrected by corresponding sensitivity factors based on Scofield photoionization cross-
sections. Analysis of the data was carried out by the software of XPS Peak 4.1 provided
by the instrument.
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2.3 Catalytic performance tests
In accordance with the typical procedure, the oxidative esterification of benzyl alcohol
was carried out in an oxygen-filled 10-mL Schlenk flask connected to an oxygen-filled
balloon (1 atm). The flask was charged with benzyl alcohol (250 mg, 2.3 mmol),
methanol (4 mL), K2CO3 (37 mg, 0.27 mmol) and catalyst (40 mg). The reaction mixture
was magnetically stirred (800 rpm) at 60 °C (temperature of thermostatic oil-bath). At
appropriate time intervals, stirring was stopped and, after catalyst settling, samples from
the reactor were analysed by GC (flame-ionization detector) on an LKhM-8MD
chromatograph equipped with 0.031.2 m metallic column with HayeSep Q sorbent
(Chrompack). With the purpose of additional control, the quantification of the analytes
was further verified using 1H NMR measurements [33].
3. Results and discussion
The reference catalyst samples denoted as Co@NC-Phen and Co@NC-ZIF, were prepared as
described in the literature, by the incipient wetness impregnation of VulcanXC72R carbon with
the Co(II) complex with 1,10-phenanthroline [6], and through a separate ZIF-67 synthesis [9,26],
respectively. The samples denoted as Co@NC-Gr were obtained from the use of dry grinding
method. Table 1 compares the catalytic performances of these samples in the oxidative
esterification of benzyl alcohol. In contrast to previous studies [6,9], the catalyst loading (relative
to benzyl alcohol) was lowered by half, to 14 wt%, even though the reaction time was reduced
from 24 to 12 h. This has enabled a more accurate comparison of the catalytic activity under
o
applied reaction conditions (atmospheric pressure of oxygen and 60 C). Also, for a number of
catalysts, benzyl alcohol conversion as a function of reaction time is shown in Fig. S1
(Electronic Supplementary Information, ESI).
It is noteworthy that the mechanochemically derived catalysts have demonstrated
considerable activity. Moreover, Co@NC-Gr3 and Co@NC-Gr7 have shown quite comparative
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or even much better performances relative to Co@NC-Phen and Co@NC-ZIF samples (Table 1,
entries 5 and 10). Only Co@NC-Gr4 sample turned out to be inactive, which, however, might
be attributed to its low porosity (Table S1, ESI).
To assess the reusability of the catalysts, several recycles were conducted with the most
active Co@NC-Gr7 sample to compare it with the recycling properties of Co@NC-Phen [6] and
Co@NC-ZIF [9]. In fact, Co@NC-Gr7 is successfully recycled without significant loss of
activity, remaining as active as a fresh sample of Co@NC-Phen, even after two runs (Fig. S2,
ESI). These results validate the achievement of the similar catalytic behavior for Co@NC
samples obtained by the simpler mechanochemical approach. Considering the lack of optimized
preparation conditions at present, we think that many possibilities exist to enhance further the
performance of Co@NC-Gr. For example, the use of the automatic grinding in the preparation of
Co@NC-Gr7 significantly improved the catalytic activity (Table 1, entry 10). Likewise, the
source of Co(II), as well as the nature of nitrogen ligand, seem to be not so essential (cf. ref. 6),
which would also facilitate elaboration of the Co@NC-Gr catalysts.
In order to gain insight into the origin of the catalytic behavior of the Co@CN-Gr samples, a
series of characterization techniques was applied to qualify the catalysts. As shown in Table 1,
neither content of cobalt, ranging from 5 to 47 w%, nor the values of total specific surface area
of Co@NC have notable effect on the conversion of benzyl alcohol and selectivity to methyl
benzoate (1) (see, however, Table S1 for the influence of meso- and micro-porosity). This may
suggest that the real active sites for the oxidative esterification are highly dispersed Co or CoNx
centers supported on carbon, in agreement with the earlier viewpoints [2,3,17,23,25,27,29].
Powder XRD was used to determine a possible difference in the composition of the large
particles in selected samples. As shown in Fig. 1, all the catalysts, including inactive Co@NC-
Gr4 sample, contain graphitic carbon (a broad peak at 25o). It should be noted that according to
XRD, VulcanXC72R carbon itself has an intermediate (turbostratic) structure between
amorphous and graphitic [34,35]. Next, the catalysts contain metallic cobalt (peaks at ~ 44, 52,
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and 76o 2theta (not shown)) and small amounts of CoO (peak at ~ 43o 2theta). The latter peak
may also be attributed to the in-plane (100) and (101) reflections of carbon [35]. The clearly
detectable particles of Co3O4 (JCPDS 42-1467) are present only in Co@NC-ZIF, and their role
as active catalytic sites is therefore doubtful.
The morphology and metal dispersion in the most active Co@NC-Gr7 catalyst and in the
“parent” Co@NC-ZIF sample were investigated using HRTEM (Fig. 2), combined with local
elemental analysis performed with EDX (Figs. S3 and S4, ESI). As shown in Fig. 2, cobalt
nanoparticles of ~ 10–30 nm are uniformly distributed in a porous graphite-like carbon matrix in
both the catalysts. The only notable difference between these samples is the higher proportion of
Co nanoparticles in the less active Co@NC-ZIF catalyst, in agreement with the relative contents
of Co measured by the X-ray fluorescence method (Table 1). Hence, the cobalt nanoparticles
observable with TEM are not intrinsically connected with the catalytic activity, thus again
supporting the view that it might be that the highly dispersed Co or CoNx centers play a decisive
role in the reaction process.
For a more detailed comparative characterization of the catalysts, XPS investigation of their
surface chemical composition was performed. The elements of Co, C, N and O could be found in
the full survey spectra (Fig. S5, ESI) of Co@NC-Phen, Co@NC-ZIF, Co@NC-Gr3 and
Co@NC-Gr7, revealing the relative contents of these elements (Tables S2 and S3, ESI). In the
Co 2p region (Fig. 3a), all the samples exhibit similar spectra consisting of two major peaks at
780.6 and ~ 796 eV, which are related to Co 2p3/2 and Co 2p1/2, and weaker satellite peaks, which
together can be assigned to oxidized forms of cobalt (Co2+). Well notable additional peak
located at ~ 778 eV is assigned to metallic Co0. From the high-resolution C 1s spectrum (Fig.
3b), three peaks at the binding energies of 284.5, 285.8 and 288.1 eV are assigned to graphitic
carbon, carbon-nitrogen bonds and C=O or COOH groups, respectively, while a weak peak at
290.2 eV can be assigned to carbonates. Regarding the N 1s spectrum (Fig. 3c), three distinct
peaks can be observed with electron binding energies of pyridinic N (398.9 eV),
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pyrrolic/graphitic N (400.8 eV), and nitrogen within the NOx functionalities (402.5, 405.4 eV). It
is found, however, that the relative contribution of these different nitrogen forms (Table S4,
ESI), and likewise different carbon species (Table S5, ESI), does not correlate with the catalytic
activity of the samples. But overall, the XPS characteristics are quite consistent with those of the
Co@NC materials obtained previously through different methods [6,9,16,27,28,29].
It is apparent that the high temperature graphitization of a carbon-containing material
(regardless if it is a carbon black or an organic compound), occurring in the presence of Co(II)
and N-heteroarenes, leads eventually to a uniform formation of the Co-N-doped graphite
composites, which are filled with a greater or lesser amount of Co-nanoparticles (cf. refs.
23,27,28). It appears that this process, not so much dependent on the initial procedures, aimed at
ensuring better deposition/distribution of the Co(II)-N complexes inside a carbon matrix. As can
be seen, in its simplest implementation, such deposition can be even performed by the physical
mixing of solid starting components without obvious negative consequences on the resulting
catalytic behavior.
4. Conclusions
We have synthesized a set of Co@NC nanocomposites through a new, atom-efficient,
solvent-free, mechanochemical approach based on simple physical mixing of solid cobalt(II)
salts, N-heteroarenes and carbon black, followed by thermal treatment. The catalytic behavior of
the obtained materials was illustrated by the example of the low-temperature oxidative
esterification of benzyl alcohol. It is demonstrated that the method developed can lead to
catalysts with activity well-compared with that of the known analogues, obtained previously
through the preliminary preparation of cobalt-nitrogen-containing MOFs or by means of
successive wet impregnation of carbon materials with cobalt salts and N-ligands. Therefore, it
could be concluded that the graphitization process rather than the preparatory stage is the key for
the successful formation of desirable Co@NC materials. Considering that besides Co itself
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many other transition metals assist the controllable carbonization/graphitization of organics [28],
a similar approach can be followed in the future for the facile preparation of related high-
performance Me@NC catalysts to promote their practical use.
Acknowledgements
This work was conducted within the framework of the budget project #АААА-А17-
117041710083-5 for Boreskov Institute of Catalysis.
Conflicts of interest
The authors declare no conflicts of interest.
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Table 1. Catalytic performance of Co@NC catalysts in the oxidative esterification of benzyl
alcohol with methanol.a
Entry
Catalyst
Cobalt salt/Lb
Carbon Pyrolysis
Co
SBET
Conversiond
(%)
Selectivityd
(%)
support (o C, h, gas) (w%)
(wt%)c
(m2 g-1)
1
2
-
1
Blanke
-
-
-
-
-
<1
76
58
57
72
1
-
2
Co@NC-Phenf
Co@NC-Gr1g
Co@NC-Gr2g
Co@NC-Gr3g
Co@NC-Gr4g
Co@NC-Gr5g
Co@NC-ZIFh,i
Co@NC-Gr6g,i
Co@NC-Gr7j,k
Co@NC-Gr8g,i
Co@NC-Gr9g,i
Co1/L1 = 1:2
Co1/L1 = 1:2
Co1/L1 = 1:2
Co1/L1 = 1:2.5
Co1/L1 = 1:5
Co2/L1 = 1:2
Co2/L2 = 1:40
Co2/L2 = 1:2
Co2/L2 = 1:2
Co2/L2 = 1:5
Co1/L2 = 1:5
65
65
25
65
25
65
-
800, 2, Ar
800, 2, Ar
800, 2, Ar
800, 2, Ar
800, 2, Ar
800, 2, Ar
850, 2, Ar
850, 2, Ar
850, 2, Ar
850, 2, Ar
850, 2, Ar
5.1
168
93
97
91
3
9
3
5.0
4
7.9
148
104
134
140
174
233
274
391
272
86 14
5
4.8
93
0
7
6
6.0
90
7
5.6
62
64
63
88
67
55
89 11
88 12
87 13
8
32.8
46.5
12.5
19.1
19.4
9
10
50
20
20
10
11
12
96
4
83 17
80 20
a
Reaction conditions: 250 mg of benzyl alcohol, 4 mL of CH3OH, 37 mg of K2CO3, 40 mg of
catalyst, 1 atm O2 (balloon), 60 oC, 12 h.
b
Molar ratio. Salts: Co1 = Co(OAc)24H2O; Co2 = Co(NO3)26H2O. Ligands: L1 = 1,10-
phenanthroline1H2O; L2 = 2-methylimidazole.
c VulcanXC72R, weight content before heat treatment.
d Determined by GC.
e Without a catalyst and K2CO3.
f The wet impregnated sample (cf. ref. 6).
g The sample is obtained by dry grinding method.
h A MOF-derived sample obtained by wet preparation method (cf. refs. 9, 26).
i The sample exhibits strong magnetic properties.
j The sample is obtained by dry grinding using a FRITSCH Mortar Grinder Pulverisette 2.
k The sample exhibits moderate magnetic properties.
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Fig. 1. Powder XRD patterns of (a) Co@NC-Phen and related samples, (b) Co@NC-ZIF and
related samples.
Fig. 2. TEM images of Co@NC-ZIF (a), (b) and Co@NC-Gr7 (c), (d).
Fig. 3. XPS of (a) Co 2p, (b) C 1s and (c) N 1s of Co@NC-ZIF, Co@NC-Phen, Co@NC-Gr7
and Co@NC-Gr3 (from bottom to top).
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Credit Author Statement
Tatiana V. Astrakova: conducting a research process, performing the experiments. Vladimir I.
Sobolev: Supervision. Konstantin Yu. Koltunov: data curation, writing and reviewing.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
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(a)
(b)
Fig. 1
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40
30
20
10
0
0
20
40
60
80
100
Size, nm
40
30
20
10
0
0
20
40
60
80
100
Size, nm
Fig. 2
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Fig. 3
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Graphical Abstract:
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• A simple, mechanochemical approach for fabricating Co@NC nanocomposites is suggested.
• The Co@NC materials thus obtained exhibit the state of the art catalytic behavior.
• The oxidative esterification of benzyl alcohol on Co@NC was used as a probe reaction.
26